Modern aircraft have horizontal stabilizers located at the tail section of the fuselage or the rudder section that are pivotally supported relative to the airplane fuselage to “trim” the aircraft during flight by selective adjustment by the operator or pilot from an internal control unit. This involves adjusting the position of the horizontal stabilizer by a stabilizer actuator to accommodate different load distributions within the aircraft and different atmospheric conditions, i.e. wind, rain, snow, etc. In this regard the stabilizer is traditionally pivotally connected to the tail section of the fuselage at a point generally midway along its length. One common trimmable horizontal stabilizer actuator consists of a primary ball nut assembly connected with an actuating drive gimbal which is pivotally connected to one end of the horizontal stabilizer structure. The ball nut assembly includes a ball nut housing and a rotatable ballscrew extending axially and usually vertically through the ball nut housing and a drive gimbal housing. The ball nut housing is connected to the drive gimbal housing by a trunnion segment. The ballscrew, in turn, may have its upper end remote from the actuating drive gimbal and may be fixed from translation or axial movement by a connection to a second, support gimbal which is pivotally secured to the tail section. As the ballscrew is rotated, the drive gimbal will be moved in translation relative to it. Thus, as the ballscrew is rotated in one direction, the leading edge of the horizontal stabilizer is pivoted upward, whereas by rotating the ballscrew in the other direction, the leading edge of the horizontal stabilizer is pivoted downward. Rotation of the ballscrew is routinely done by a motor and associated gearing which is connected to the second, fixed support gimbal and which is actuated by the operator or pilot by the internal control unit. The connection of the stabilizer actuator to the stabilizer is often located within the fuselage tail section and not directly in the air stream.
The horizontal stabilizer movement, as controlled by the operator, is transmitted by the ballscrew through the actuating drive gimbal by way of the primary ball nut assembly which defines a primary load path. The movement has a load with tensile and compressive components as well as a torque component due to the ballscrew thread lead. Failures of the primary load path such as caused by the shearing off of the connecting trunnion segment or by the loss of nut ball members from the ball nut assembly can result in the complete loss of control of the horizontal stabilizer. However, stabilizer actuators have always been provided with a secondary load path as a protection against the catastrophic failure of the primary path (ball screw or its attachment points).
For safety, the structural failure of any member of the primary structural load path should be annunciated within a single flight and preclude a subsequent dispatch of the aircraft.